As electronic components become increasingly miniaturized, it becomes more difficult to build and model them. This is especially true with respect to manufacturing and simulating miniature microwave devices for several reasons.
First, computer simulations of microwave devices alone are typically not adequate. Instead, actual prototypes must be built to make sure the device functions as intended. This is because such microwave systems generally contain several devices of equivalent complexity and the inherent limitations of simulation software. This dramatically increases total prototyping time to several weeks or months. Second, at microwave frequencies, the difficulties of soldering components to microstrip traces can create an uncertainty at the direct electrical connection that is difficult to characterize and model. Third, when various electronic components are positioned close together on a board, parasitic inductances and capacities and time delays with signal propagation occur. All of this creates circuit design problems when the circuit elements are positioned tightly together in a small area (because the physical circuit elements can interfere with microwave radiation and with each other). Fourth, with microwave devices, the sizes of the electronic components are often within an order of magnitude of the size of the signal wavelength. As such, microwave signals may have a wavelength that is roughly the same length as the size of the physical circuit elements themselves. All of these effects make circuit modeling difficult.
Traditional microwave circuits are produced using printed circuit board technology as is shown in FIG. 1. Such circuits are typically composed of a circuit board 10 (being a low loss dielectric material), and a patterned layer of metal 12 on top (having a very high electrical conductivity). The patterning of metal layer 12 largely determines the response and quality of the microwave device. In FIG. 1, the gap 15 between two pieces of metal is the “critical gap spacing.” For the device of FIG. 1 (a band-pass filter), the critical gap spacing 15 determines how well the filter passes energy between “signal input” at end 14 and “filtered signal output” at end 16.
The exemplary device seen in FIG. 1 (and many similar devices like it) are actually quite complex to manufacture and typically require many design iterations before acceptable performance levels are achieved because of the challenges listed above. Even the best available simulation technologies are not able to entirely predict how the design will respond. Therefore, to reach acceptable levels of performance, several values for critical gap 15 are typically chosen and a number of prototypes are sequentially manufactured and tested until the desired performance is reached. This process could easily take several days or weeks, and is extremely costly. Thus, even with powerful design software, the engineering of microwave circuits usually require cut-and-try engineering using old fashioned knives, copper tape and an iterative procedure.